1. Field of the Invention
The present invention relates to a method of producing and treating glass strands which are intended, in particular, for reinforcing organic substrates, the glass strands being coated with an essentially organic sizing composition which can be polymerized under the action of heat. The present invention also relates to the glass strands produced according to this method and to the composites produced from the strands.
2. Discussion of the Background
The direct manufacture of glass strands is carried out in known manner from threads (or "thin streams") of molten glass which flow from the holes in one or more dies. The threads are drawn mechanically to form continuous filaments and the filaments are then assembled into one or more basic strands, the strands then being collected on moving supports or undergoing other treatments before collection, according to the required applications.
Before they are assembled in the form of strands, the filaments are coated with a sizing composition by passing over a continuously-supplied sizing member. This deposition of sizing composition is necessary for the production of the strands and enables the strands to be associated with other substances, such as organic substrates, to produce composites.
Indeed, in the first place, the sizing composition acts as a lubricant and protects the strands from abrasion resulting from friction of the strands on various members such as guide members, at high speed, during the process mentioned above.
In most cases, the sizing composition also ensures the integrity of the aforementioned strands, that is, the bonding together of the filaments of which they are constituted. This integrity is required particularly in textile applications where the strands are subject to large mechanical stresses. Indeed, if the filaments constituting the strands are not bound to one another, they break more easily in the course of such applications and clog the textile machines. In general, integrity of the strands is required, strands without integrity being considered difficult to handle and, in particular, difficult to extract from windings manually or by means of machines.
The sizing composition also enables the strands to be associated with various substrates to be reinforced in order to produce composite elements, by helping to create bonds between the strands and the substrates to be reinforced and facilitating the wetting of the strands by the substrate. The wetting of the strands means the spreading of the substrate on the surface of the strand, but also its penetration into the interior of the strand which is composed of filaments. The mechanical properties of the composites produced from the substrate and the strands depend, in particular, upon the quality of the adhesion of the substrate to the strands and the capacity of the strands to be wetted by the substrate. The wettability characteristics conflict, however, with the characteristics of integrity mentioned above. In fact, the greater the integrity of a strand, the stronger is the bonding of the filaments to one another and the more difficult it is for the substrate to be reinforced, to penetrate the interior of the strand. In fact, existing sizing compositions are generally the result of a compromise between the capacity to reinforce other substrates and the integrity required in the strands.
In order to be suitable for the methods used, the sizing composition must, in particular, be stable and compatible with the drawing speeds of the filaments which have to pass through it. The sizing composition should not be the source of excessive shearing phenomena due to high speeds of the filaments (which are drawn at a speed of several tens of meters per second) and should not pose problems in wetting the surfaces of the filaments at the speeds used. In methods which make use of steps for the heat treatment of the sizing composition, it is convenient, moreover, to use sizing compositions which react thermally and have a high enough reaction temperature to remain stable under the die.
Although the selection of the method involves the use of a sizing composition which responds to certain conditions, the selection of the sizing composition may also influence the method.
Thus, most sizing compositions used at the moment, are aqueous sizing compositions which are easy to use but which comprise large quantities of water (90% by weight of the sizing composition, leading, in particular, to windings having from 8 to 14% by weight of water) which necessitates the provision of a step for drying the strands before any use of the strands for reinforcing organic substrates since the water may detract from the good adhesion between the strands and the substrates. These drying steps are long and costly. When they are carried out during the drawing operation (that is, before the collection of the strands obtained after the assembly of the filaments) either at the level of the filaments (WO 92/05122) or at the level of the strands (U.S. Pat. No. 3,853,605), they necessitate the installation of drying devices under each die and, when they are carried out on windings of strands, they involve the risk of migration of the sizing composition into the interior of the windings. These migrations occur in a more or less selective manner according to the affinity of the sizing components for glass and they lead to the production of strands having characteristics which are not uniform along their lengths. Aqueous sizing compositions are moreover distributed unevenly on the strands from their deposition, due to their nature. In addition to these migration phenomena, in certain cases, there are phenomena of coloration of the strands or deformation of the windings. Moreover, the effectiveness of the drying steps is not always optimal, these drying steps having to be adapted to the various manufacturing conditions and depending on the composition of the sizing composition, the weight of the strands to be dried, etc..
A few patents propose solutions for reducing the drying and/or migration problems with the use of non-aqueous sizing compositions but the sizing compositions in question generally make use of organic solvents which are difficult to handle and may be harmful to the health of people in the vicinity because they are toxic and/or may pose problems of viscosity which can conveniently be solved by heating the sizing compositions (U.S. Pat. No. 4,604,325) or by adding suitable agents (U.S. Pat. No. 4,609,591). The use of these sizing compositions in many cases also necessitates the installation of certain devices below each die; in particular, it is necessary, when the strands coated with the sizing compositions are collected in the form of a winding, to treat the strands before producing the finished winding to prevent excessive bonding between the strands, since this bonding makes it difficult to unwind the strands. The treatments consist, for example, of heating the layer of continuous filaments coated with the sizing composition so as to eliminate the solvent of the sizing composition before assembling the filaments (U.S. Pat. No. 3,425,862) or polymerizing the sizing composition by subjecting the strand to the action of ultraviolet rays over at least a portion of its travel (U.S. Pat. No. 5,049,407). The effectiveness of these treatments depends, once again, on the operating conditions.